Feeding biology of yellowfin tuna (Thunnus albacares) in tropical central and western Pacific Ocean

We had collected 919 individuals of yellowfin tuna (Thunnus albacares) in the tropical central and western Pacific Ocean (163°14'E−173°35'E, 2°03'S−11°17'S) from May 2018 to February 2019 by pelagic longline fishing trageting tunas to analyze their size frequency and sex ratio. The feeding biology of 551 individulas was also studied. The effects of fork length, fishing water depth and lunar phase on feeding biology were analyzed by analysis of variance (ANOVA) and generalized linear mixed model (GLMM). The results show that: 1) The fork length range was 87−163 cm, and the sex ratio (female to male) was 1: 1.32. 2) The hollow stomach rate accounted for 33.76%. For tunas with non-empty stomach, the frequency of presense of sardines, squid and miscellaneous fish in stomach contents were 52.60%, 35.34% and 33.70%, respectively. 3) The ANOVA results show that fork length and lunar phase had significant effects on the Shannon-Weiner diversity index (H') and Pielou evenness index (J'), while month and water depth did not. 4) The feeding intensity was mainly at Level 0−2, and the empty stomach rate increased as the fork length increased. The proportion of high feeding intensity (Level 2−4) increased gradually as water depth increased. 5) The result of GLMM fit shows that biological factors (fork length and gender), spatial and temporal factors (cosine of lunar phase and water depth) had significant effects on the feeding intensity of yellowfin tuna.